In image forming apparatuses, such as printers, facsimiles, copy machines, multifunctional machines, or the like, a liquid ejection device including a recording head or liquid ejection head is used to perform image formation (i.e., recording, printing, photo-printing, or character-printing) using recording liquid or ink. Commonly, such a recording head includes a plurality of nozzles for ejecting ink droplets, with which image formation is performed by ejecting and depositing ink onto a recording medium or recording sheet supported and moved on a media transferring member such as a transfer belt.
Note that “image forming apparatus” hereby refers to an apparatus that performs image formation by depositing recording liquid onto a medium such as paper, thread, fiber, cloth, leather, metal, plastic, glass, wood, ceramics, etc., and includes inkjet printers, textile printers, wiring circuit printers, and the like. Also, the term “image formation” refers to formation of images on recording media, including those with meanings, such as characters, pictures, etc., as well as those without concrete meanings, such as designs, patterns, etc. It should be noted that the recording liquid is not particularly limited and includes any liquid used for image formation.
Occasionally, recording heads used in image forming apparatuses suffer a nozzle defect, where a nozzle cannot properly eject droplets due to defects such as clogging with ink, etc. Since such a defect leads to degradation of image quality, e.g., white lines appearing on formed images, it has been a common practice to detect whether a recording head has a defective nozzle, and to restore the image forming apparatus to proper working condition upon detection of a nozzle defect.
Various methods have been developed to detect a nozzle defect in image forming systems. In one method proposed, a test pattern of dots made of cyan ink, magenta ink, and yellow ink is formed in a given region on the surface of a sheet transfer member. According to this method, the dot pattern is read by an RGB sensor, and a defective nozzle is detected based on an output of the RGB sensor.
Another detection method proposed includes a read unit for reading a test pattern, which is an image formed on a transfer member for holding and transferring a recording medium.
In addition, there has been a detection method for use in an electrophotographic image forming apparatus that uses toner for image formation, where density of a formed image is determined based on an output of a light sensor. The light sensor can simultaneously sense specular light and diffused light reflected from an image, which indicates the amount of toner adhering to a recording medium.
However, when using a test pattern formed on a transfer member for transferring a medium, for example, on a transfer belt as in the above methods, it is difficult to accurately detect the test pattern by identifying colors or by photographing, since, depending on the combination, a color difference between the test pattern and the transfer member can be too small to detect by the read unit. In this case, accurately detecting respective colors requires an expensive detection means such as light sources having different wavelengths for different colors.
Moreover, when using an electrostatic transfer belt having a front surface formed of an insulation layer and a back surface formed of a medium resistant layer to which carbon is blended to provide sufficient electric conductivity, it is difficult to accurately detect the test pattern by sensing a color difference or by photographing since the electrostatic belt is black in color and is hardly discernible from black ink.
In the above-mentioned detection method using the RGB sensor, detection accuracy is deteriorated when the color of an ink droplet to be ejected is similar to that of the transfer member. Therefore, a good detection accuracy is obtained only with limited variations of color inks for a particular transfer member to form the test pattern thereon. Further, when configuring the RGB sensor using a laser that has a significantly tiny spot diameter, detection accuracy is lowered when small foreign matters or scratches on the transfer member affect the laser scanning performance. Such a method is also disadvantageous in terms of cost, since the RGB sensor requires multiple elements for reading respective colors.
To cope with the above problem, it is considered to apply the above-mentioned detection method for use in an electrophotographic system to an inkjet printing system. However, directly applying such a method cannot achieve accurate detection of an ink pattern. An electrophotographic system can perform pattern detection using the test pattern according to the detection method in which toner particles, which remain stable in shape when in contact with each other, are collected and piled up in a rectangular line. By contrast, liquid droplets tend to aggregate when disposed in contact with each other, so that it is difficult, if not impossible, to detect a test pattern formed by closely depositing ink droplets, and detection using such a test pattern provides an output that cannot be distinguished from noise.
Further, when the test pattern is formed on ink-permeable plain paper, ink penetrates the plain paper and smudges, making obscure the test pattern. This also poses a difficulty in accurately detecting a defective nozzle in an inkjet image forming apparatus.